Portland Mayor Sam Adams is proposing using money from sewer and garbage to pay for a $55 million plant to turn food waste into electricity.

The plan, taking shape behind closed doors but outlined in a memo obtained by The Oregonian, calls for Columbia Biogas, a private company, to build the plant in Northeast Portland's Cully neighborhood.

Under one version of the plan, the city would backstop a private loan to the company, offering what's called credit enhancement that could require the city to kick in as much as $900,000 a year for up to 20 years.